The primary goal of the proposed research is to elucidate the mechanism of host innate immune responses and viral counter-defense responses at an epigenetic level. Multi-cellular eukaryotes including humans and plants have evolved sophisticated and fundamental mechanisms of RNA silencing to defense invasive viruses. On the other hand, viruses including those infecting humans encode proteins, referred to as viral suppressors, to block silencing pathways to evade host surveillance. The current world-wide study on host-virus interaction focuses on the antivirus role of posttranscriptional gene silencing (PTGS) and viral suppression of PTGS. While our knowledge of viral suppression at the PTGS level has been drastically expanded, our understanding of viral suppression at the level of transcriptional gene silencing (TGS) is still at its infancy. The preliminary studies from the PI's group have illuminated that AL2 suppressor encoded by geminivirus, a family of single-strand DNA viruses, genetically interferes with the pathway of Heterochromatin Protein 1 (HP1) and Polycomb Repressive Complex 2 (PRC2), which are critical genes responsible for epigenetic silencing. Moreover, AL2 biochemically interacts with HP1. These results and work from several other groups led to conceptualization of a model that TGS serves as an innate immune system to restrict invasive DNA pathogens, whereas viral suppressors can break this restriction by directly inhibiting the TGS integrators. The proposed study will address this model: Aim1 will test the hypothesis that HP1/PRC2 is a new line of host antivirus system and AL2 interaction with these components represents a novel counter-defense strategy. Effect of loss-of-function and gain-of-function mutations of HP1/PRC2 genes on modification of geminivirus chromatin will be investigated. Aim 2 will address a speculation that, through interaction with HP1/PRC2, AL2 can reprogram host gene expression and trigger downstream pathogenesis signaling cascades that disrupt host cellular environment leading to disease. A total of 30 AL2-responsive genes have been selected and whether nor not these genes are involved in AL2-elicited pathogenesis processes will be investigated. The proposed study will serve as the starting point for further comprehensive studies on the molecular mechanism of pathogenesis triggered by viral suppressors from the DNA viruses. Understanding these mechanisms will elucidate basic events that occur at the interface between virus and host, and will reveal natural antiviral defense strategies that may be exploited for directed therapies or preventative measures to address health disorders that arise from epigenetic dysfunction in mammalian hosts. PUBLIC HEALTH RELEVANCE: Multi-cellular eukaryotes including humans and plants have evolved sophisticated and conserved epigenetic silencing mechanisms to protect themselves from virus attack. The proposed study is to elucidate the mechanisms by which geminivirus-encoded AL2 protein suppresses epigenetic silencing and triggers downstream pathogenesis signaling cascades that lead to diseases. The proposed study on Arabidopsis immune responses to infection by model DNA viruses and the viral counter- defensive strategies will provide mechanistic insights into the molecular interactions between viruses and mammalian hosts.